The present invention relates to a semiconductor device having capacitive elements and semiconductor devices having metal interconnections and ferroelectric capacitance or high dielectric capacitance.
In recent years, developments of ferroelectric memories utilizing ferroelectric capacitances and dynamic random access memories utilizing high dielectric capacitances have been active. Those ferroelectric memories and dynamic random access memories have switching transistors such as MOS field effect transistors. The ferroelectric capacitor or the high dielectric capacitor is electrically connected to one of the diffusion regions, for example, the source/drain diffusion regions so that the ferroelectric capacitor or the high dielectric capacitor can act as a memory cell in the ferroelectric memory or the dynamic random access memory.
The ferroelectric capacitor has a capacitive dielectric film made of a ferroelectric material, for example, Pb(Zr, Ti)O3, hereinafter referred to as xe2x80x9cPZTxe2x80x9d. Upon application of an electric field to the ferroelectric capacitive film, the ferroelectric capacitive film shows a polarization which enables the ferroelectric capacitor to act as a non-volatile storage.
The high dielectric capacitor has a capacitive dielectric film made of a high dielectric material, for example, (Ba, Sr)TiO3, hereinafter referred to as xe2x80x9cBSTxe2x80x9d. A high dielectric constant of the high dielectric film increases a capacitance of the high dielectric capacitor. This increase in capacitance of the high dielectric capacitor enables the capacitor to be scaled down.
For use of the ferroelectric capacitor or the high dielectric capacitor, it is necessary that one of electrodes of the ferroelectric capacitor or the high dielectric capacitor be connected to one of the source/drain diffusion regions of the switching transistor, for example, the MOS field effect transistor.
In the dynamic random access memory, a bottom electrode of the capacitor may be made of a polysilicon, wherein the bottom electrode is connected to one of the source/drain diffusion regions of the switching transistors. The dielectric film is then formed on the polysilicon bottom electrode. The ferroelectric capacitive film and the high dielectric capacitive film are made of oxides, for example, Pb(Zr, Ti)O3, and (Ba, Sr)TiO3, respectively. For this reason, if the ferroelectric capacitive film or the high dielectric capacitive film is formed on the polysilicon bottom electrode, then a top surface of the polysilicon bottom electrode may be oxidized by the ferroelectric capacitive film or the high dielectric capacitive film. The oxidation of the bottom electrode is of course a problem. In order to attempt to solve the above problem of oxidation of the surface of the polysilicon bottom electrode with the ferroelectric capacitive film or the high dielectric capacitive film, the following conventional methods had been proposed.
The first conventional method is disclosed in 1995 Symposium On VLSI Technology Digest Of Technical Papers, pp. 123, which describes a cell structure wherein a top electrode of the capacitor is connected to the diffusion region of the switching transistor through a metal interconnection.
The second conventional method is disclosed in 1994 International Electron Devices Meeting technical Digest, pp. 831, which describes that an oxide dielectric film of SrTiO3 is formed on an RuO2/TiN bottom electrode formed on a polysilicon plug.
In accordance with the conventional methods of forming the ferroelectric memory and the dynamic random access memory, the capacitor is formed, before a metal interconnection is formed.
The above memory cell structure using the local interconnection or polysilicon plug connecting the capacitor to the diffusion region of the switching transistor has the following five problems.
The first problem is in the difficulty to form multilevel metal interconnections. It is necessary to form multilevel metal interconnections in order to realize a high density integration of the ferroelectric memories having the ferroelectric thin films or the dynamic random access memories having the high dielectric thin films, or to realize a composite semiconductor device having such memory device and logic circuits. For forming the multilevel interconnections, an inter-layer insulator between adjacent different level interconnections is planarized by a chemical mechanical polishing method. However, the existence of the capacitor makes a difference in the level of the inter-layer insulator between a memory cell array area having the capacitors and a logic circuit area free of any capacitor. This difference in the level of the inter-layer insulator makes it difficult to planarize the surface of the inter-layer insulator. Otherwise, the difference in the level of the inter-layer insulator makes it difficult to connect the adjacent different level interconnections to each other through a via hole or connect the first level interconnection to the diffusion region of the transistor through a contact hole but after the planarization to the inter-layer insulator. In Japanese laid-open patent publication No. 9-92794, it is disclosed that a difference in the level of the inter-layer insulator between a memory cell area and a peripheral circuit area is reduced to form multilevel interconnections with reduced resistances on the peripheral circuit area. In accordance with this conventional method, after the via hole or the contact hole has been formed in the inter-layer insulator, then an electrically conductive material is deposited to fill the via hole or the contact hole and also to overly the inter-layer insulator, before the deposited conductive material is selectively etched to leave the same only within the via hole or the contact hole, whereby the surface of the inter-layer insulator is showing. It is, however, difficult to realize a highly accurate etching to the electrically conductive material so that the electrically conductive material remains only in the contact hole or the via hole without any over-etching to the surface of the inter-layer insulator.
The second problem is in an increase in the design cost for realizing the hybrid semiconductor integrated circuits having logic circuits and semiconductor memories. In order to settle the above first problem, it is required to change the process and device of the logic circuits. This means that the design parameters already used are no longer usable for other processes.
The third problem is the deterioration in electrical characteristics of the capacitor due to the process for forming the multilevel metal interconnections. Normally, a tungsten plug is formed in a via hole for connecting the adjacent different level metal interconnections. The tungsten film may be formed by utilizing the following chemical reaction.
2WF6+3SiH4xe2x86x922W+3SiH4+6F4.
The formation of the tungsten film is carried out in an extremely strong reduction atmosphere. Since the ferroelectric thin film and the high dielectric thin film are made of oxides, exposure of the ferroelectric thin film and the high dielectric thin film to the reduction atmosphere causes an oxygen deficiency of the ferroelectric thin film or the high dielectric thin film as the capacitive thin film, whereby a resistance of the capacitive thin film is dropped whilst a leakage of current across the capacitive thin film increases. The oxygen deficiency further causes reductions in polarization of the ferroelectric film and in dielectric constant of the high dielectric film. The oxygen deficiency deteriorates the electrical characteristic of the capacitor.
In Japanese laid-open patent publication No. 9-199679, it was proposed to avoid the use of the reducing atmosphere for burying a metal into a deep contact hole. Plug contacts made of a thermally stable metal have been formed within openings which reach diffusion regions of CMOS circuits and memory circuits, before a ferroelectric capacitor is formed which is connected through some of the plug contacts to the diffusion regions, and further aluminum interconnections are formed in contact with the remaining ones of the plug contacts. The formation of this structure involves complicated processes. This conventional structural feature is applicable but only to the first level metal interconnection.
The fourth problem is the deterioration of characteristics of the switching transistors, for example, variation in threshold voltage of the switching transistors and deterioration in sub-threshold characteristic of the switching transistor. A gate oxide film of the MOS field effect transistor may receive substantial damage from plasma during the plasma etching process, whereby interface states and fixed charges are formed in the gate oxide film of the MOS field effect transistor. Those interface states and fixed charges cause variations in threshold voltage of the switching transistors and deterioration in sub-threshold characteristic of the MOS field effect transistor.
In order to solve the above fourth problem, a hydrogen anneal in an atmosphere containing hydrogen has been carried out. If, however, this anneal, as the heat treatment, is carried out after the ferroelectric capacitor having the ferroelectric thin film or the high dielectric capacitor having the high dielectric thin film has been formed, then a diffusion of hydrogen may be caused whereby the ferroelectric thin film or the high dielectric thin film may be exposed to hydrogen. This exposure of the ferroelectric thin film or the high dielectric thin film to hydrogen causes the oxygen deficiency of the ferroelectric thin film or the high dielectric thin film as described in the third problem, whereby the resistance of the capacitive thin film is dropped whilst the leakage of current across the capacitive thin film increases. The oxygen deficiency further causes reductions in polarization of the ferroelectric film and in the dielectric constant of the high dielectric film. The oxygen deficiency deteriorates the electrical characteristics of the capacitor.
In Japanese laid-open patent publication No. 7-111318, it is disclosed that a hydrogen barrier layer such as an Si3N4 layer is provided over the capacitor to prevent diffusion of hydrogen toward the ferroelectric thin film or the high dielectric thin film, whereby deterioration of the capacitor is prevented.
This technique may, however, raise another problem with increase in manufacturing cost due to additional processes for forming the hydrogen barrier layer.
Further, responsive to the requirements for increasing the density of integration of the capacitors and scaling down of the capacitor, it is required to reduce the thickness of the hydrogen barrier layer. However, the reduction in thickness of the hydrogen barrier layer results in reduction in barrier ability. This technique is irresponsible to the advanced and future semiconductor devices in consideration of the fact that the importance for reducing variations in characteristics of the devices has been on the increase, as the scale of the integrated circuits is further enlarged and high speed performances of the integrated circuits is further improved and also individual devices are scaled down.
The fifth problem is the deterioration of characteristics of the capacitor and in an increase in resistance of the interconnection between the capacitor and other devices. This fifth problem is raised when a contact between the different level metal interconnections or a contact between the first level metal interconnection and the semiconductor substrate is formed after the capacitor has been formed. After the contact plug is formed in an inter-layer insulator for connecting the substrate to the first level metal interconnection, an ion-implantation to the contact plug is carried out to reduce a resistance of the contact plug between the substrate and the first level metal interconnection. After the ion-implantation into the contact plug, a heat treatment is further carried out at a temperature of about 70xc2x0 C. or higher for activation of the implanted ions in the contact plug. The heat treatment is thus carried out, after the ferroelectric capacitor or the high dielectric capacitor has been formed. Such heat treatment, however, may cause an inter-reaction and an inter-diffusion between the ferroelectric oxide material of the ferroelectric capacitive thin film or the high dielectric capacitive thin film and the electrode or the metal interconnection. Such inter-reaction and inter-diffusion cause deterioration of the electric characteristics of the capacitor and also cause an increase in resistance of the metal interconnection.
In the prior art, there was neither ferroelectric memory devices nor high dielectric capacitance dynamic random access memory devices, which have a multilevel metal interconnection structure but is free from any of the above five problems.
Due to the above circumstances, it had been required to develop a novel semiconductor memory device having a ferroelectric capacitor and a multilevel interconnection structure free from the above five problems, and also develop another novel semiconductor memory device having a high dielectric capacitor and a multilevel interconnection structure free from the above five problems.
The present invention provides a capacitive element structure in a semiconductor device having an interconnection structure, the capacitive element structure comprising a capacitive element having a capacitive dielectric film made of an oxide compound, wherein the capacitive element structure is positioned above at least a bottom level interconnection of the interconnection structure.
The above and other objects, features and advantages of the present invention will be apparent from the following descriptions.